The present invention relates to blends comprising polyphenylene ether and diene based rubber which exhibit improved resistance to loss of impact strength when the blends have been molded and thereafter thermally recycled. More particularly, the present invention relates to blends of diene based rubber and polyphenylene ether, where the polyphenylene ether used in the blend has been previously extruded, prior to melt extrusion with the diene based rubber.
Prior to the present invention, antioxidants, such as hindered phenols, were often used as stabilizers for polyphenylene ethers, while metal deactivators/antioxidants were frequently employed to reduce metal catalyzed degradation in unsaturated rubber. Commercially available stabilizers have been used with butadiene copolymers, such as Kraton KD1102 manufactured by Shell Chemical Company, which is a styrene-butadiene-styrene (SBS) block copolymer. Antioxidants also have been found to minimize crosslinking of diene based rubber resulting from oxidation during high temperature processing. In the event of excessive crosslinking prior to or during molding, diene based rubber can become ineffective as an impact modifier for thermoplastics, such as polyphenylene ether.
Although metal deactivators and/or antioxidants have been found effective for stabilizing diene based rubber, experience has shown that blends of such stabilized diene based rubber and polyphenylene ether which have been molded, often experience a reduction in toughness when thermally recycled, as compared to the blend when initially molded. One possible explanation is that crosslinking occurs in the diene blocks of the impact modifier during the subsequent melt extrusion steps of the recycled thermoplastic, and the toughness of the molded thermoplastic is reduced. Metal residues such as copper in the polyphenylene ether also can catalyze the crosslinking of the unsaturated rubber. Accordingly, in order to maintain the impact properties of molded blends of polyphenylene ether and diene based rubber subject to recycling, additional methods to improve the thermal stability of the diene based rubber subject to recycling are constantly being sought.